Communication between packet switching networks is done by establishing a logical channel between two packet switching networks and then using logical links within this logical channel to establish communication between digital terminals attached to both packet switching networks. Each pair of digital terminals communicating through the packet switching networks has one logical link assigned to them. This logical link time-shares with other logical links on the logical channel. ISDN standard Q.931 defines the packet protocol which is utilized by packet switching networks to establish multiple links over the same logical channel. The result is that there can be a number of packet calls between pairs of digital terminals sharing the same logical channel. A circuit switching system establishes one logical channel for each voice or data call which it is currently switching. The reason for this is that a common bandwidth voice call cannot share a logical channel with other voice calls because of the amount of bandwidth required for a voice call. ISDN standard Q.931 defines the protocol which is used by circuit switching systems to establish logical channels through a network made up of a number of circuit switching systems.
The problem that arises is when it becomes desirable to interconnect two packet switching networks with a circuit switching system. The problem is that a circuit switching system only allows one call per channel according to the ISDN standard and does not recognize requests for logical links on a logical channel. When the packet switching network needs a logical link the circuit switching system will assign this logical link to a logical channel; and upon a second request for another logical link via the same logical channel, the circuit switching system denies the second request. The second request for a logical channel is a request for a second call. The end result is that a great deal of bandwidth is wasted when using a circuit switching system to interconnect packet switching systems.
Whereas it is possible to have systems made up entirely of packet switching networks, the large embedded base of circuit switching networks in both private communication systems and in the public telephone network make this a crucial problem.
A similar problem exists with respect to subrate data calls. In sutrate data, two data multiplexers multiplex a logical channel into time slots and, then, place data calls in those time slots. Data calls are established by one data multiplexer requesting the establishment of a data call. Voice concentrators function in a similar manner by placing compressed voice calls in time slots of a logical channel. A circuit switch interposed between either data multiplexers or voice concentrators establishes a logical channel for each request. The result is that each compressed voice or subrate data call still requires an individual logical channel.